This invention relates to a novel method for producing a unique machine-readable marking on each of a plurality of workpieces. The novel method is particularly useful for producing unique bar-code markings on glass parts for cathode-ray tubes.
U.S. Pat. No. 4,374,451 issued Feb. 22, 1983 to W. R. Miller discloses a method for assembling parts of a CRT (cathode-ray tube) that includes providing at least one CRT part, such as the glass faceplate panel, having a unique machine-readable marking, such as a bar-code marking, on an external surface thereof. The marking is read one or more times by machine during the assembly of the CRT. Each time it is read, a control signal is generated in response to the reading, and then the signal is used to initiate a local process for action with respect to the tube part. The local process may be one or more of selecting and assembling another part to the workpiece, a series of processing steps applied to the workpiece, a recording of historical data, etc. The marking must be made reliably at low cost, must be readable reliably at low cost, and must survive the hostile environments of subsequent processing.
U.S. Pat. No. 4,327,283 issued Apr. 27, 1983 to P. M. Heyman et al. discloses a workpiece with a desired unique machine-readable marking recessed therein. That marking may be made by depositing a coating on a surface portion of the workpiece, and then selectively removing, as by abrasion or ablation, defined areas through the coating. The marking is then read to verify that the desired mark has actually been produced. It is desirable to provide an alternative method for producing such unique machine-readable markings, especially a method which may require less process control and can produce markings at lower cost with equivalent or better reliablity.